Measuring and displaying the passage of time must be as precise as possible and has been a historical concern. That concern is even more important for modern society with its complex economic activities and with its different communication systems which require a uniform and additionally an especially exact time measuring. Precise time information has been made possible by modern cesium controlled atomic clocks which measure the time with a very high precision. However, there is a need to provide the precise time information also to people who do not have access to an atomic clock.
Following the discovery of radio waves it has been recognized early that such waves are a suitable medium for the transmission of time information. In spite of the importance that the transmission of time information through satellites has achieved, the distribution of time information by long wave radio transmission plays an important role today. Particularly the following characteristics of a long wave radio wave make these waves of interest for the transmission of time information today and in the future. Long wave time signals have a very large range. These waves penetrate into buildings and they can be received with very small ferrite antennas. Obstructions such as trees or buildings cause a substantial damping of received high frequency satellite signals. Contrary thereto the receipt of long wave signals is hardly impaired by such obstacles. The propagation characteristics of long waves modulated with time information combined with the availability of modern microelectronics make it possible to construct compact radio controlled clocks which can, for example, be operated with a battery or by solar energy collected by solar cells.
A transmitter is required for transmitting long waves modulated with the time information. Such a transmitter must supply the respective time information to all radio controlled clocks present within the transmitting range of the transmitter. In Germany the “Physikalisch Technische Bundesanstalt (PTB)” maintains a long wave transmitter DCF-77 which transmits a reliable time signal and a normal frequency. This DCF-77 transmitter in Germany is controlled by atomic clocks and emits at the frequency of 77.5 KHz with a transmission power of 50 KW. This frequency is outside the receiver frequency range of normal radio receivers. Further, the transmitter DCF-77 transmits, on a continuous basis, long wave time signals in accordance with the official atomic time scale CET (Central European Time). Similar transmitters are used in other countries, for example in Japan and the USA. These transmitters transmit time information on long wave carrier frequencies in the range between 40 and 120 KHz by means of amplitude modulated signals.
The above mentioned countries use the same format for the transmission of time information. The so-called format is a time frame which has a duration of precisely one minute. This time frame comprises values or information for the minute, the hour, the day, the weekday, the month, the year, and so forth. This information within the time frame is present in the form of a BCD (binary coded decimal) code. These binary coded decimal codes are transmitted with a pulse duration modulation at the rate of 1 Hz per bit. In such a pulse duration modulation either the leading edge or flank or the trailing edge or flank of the first pulse of a time frame is synchronized with exactly zero or null seconds. A typical radio controlled clock is so constructed that the time adjustment begins with the point of time or instant at which the null second signal was received for the first time and then receiving the time information of a time frame or of several time frames.
The transmission of time information is accomplished by these transmitters with the aid of so-called time signals. Therefore, these transmitters or receivers of time signals are referred to herein as time signal transmitters or time signal receivers. A time signal is defined as a transmitter time signal of short duration which has the purpose or object to transmit the time reference information provided by the time signal transmitter. More specifically, the time signal is a modulation oscillation with generally several time markers which, after demodulation, merely represent one impulse or pulse which reproduces the transmitted time reference with a defined uncertainty. This demodulation result in the form of a single impulse or pulse must not be confused with an encoded time information which transmits the text of a clock time value in the form of an impulse code.
FIG. 1 shows a coding scheme or plan A of the encoded time information for the time signal transmitter DCF-77 in Germany. The coding plan or scheme comprises 59 bits whereby each bit corresponds to or represents one second of the time frame. Thus, within a minute it is possible to transmit a so-called time signal telegram which contains binary encoded information particularly with regard to time and date. The first fifteen bits B comprise a general or common encoding which for example contains operating information. The next five bits C contain a general information. The reference character R designates an antenna bit. A1 designates an announcement bit for the transition from Central European Time (CET) to Central European Summer Time (CEST) and back again. Reference characters Z1 and Z2 designate time zone bits, A2 designates an announcement bit for a leap second while S designates a starting bit of the encoded time information. Beginning with bit 21 and ending with bit 59 the time and date informations are transmitted in a BCD-code whereby the data are respectively valid for the next following minute. Further, the bits in the zone D comprise information regarding the minute, the bits in the zone E contain information regarding the hour, the bits in zone F contain information regarding the calendar day, the bits in the zone G contain information regarding the day of the week while the bits in the zone H contain information regarding the month and the bits in the zone I contain information regarding the calendar year. These informations are available in a bit-by-bit fashion and in encoded form. At the end of each zone D, E and I so-called testing bits P1, P2, P3 are inserted. The 60th bit is not occupied and serves for the purpose of indicating the beginning of the next time frame. Reference character M designates a minute marker and thus the beginning of the time information telegram.
The structure and the bit occupancy of the encoded plan or scheme A shown in FIG. 1 for the transmission of time signals is generally known and described for example in an article by Peter Hetzel entitled “Time Information and Normal Frequency” published in Telecom Praxis, Volume 1, 1993.
The transmission of the time signal information is performed by amplitude modulating a carrier frequency with the individual second markers. The modulation comprises a lowering X1, X2 or a raising of the carrier signal X at the beginning of each second with the exception of the 59th second of each minute. The carrier amplitude is reduced for the duration of 0.1 seconds at X1 or for the duration of 0.2 seconds at X2, to about 25% of the normal amplitude. These amplitude reductions and increases are used in the above mentioned transmitter DCF-77. These amplitude reductions having differing time durations respectively define second markers or data bits. These differing time durations of the second markers serve for the binary encoding of the time of day and the date, whereby the second markers having a duration of 0.1 seconds (X1) correspond to the binary “0” while those second markers with a duration of 0.2 seconds (X2) correspond to the binary “1”. The absence of the 60th second marker announces the next following minute marker. This announcement in combination with the respective second makes possible an evaluation of the time information transmitted by the time signal transmitter. FIG. 2 shows by way of an example a section or portion of such an amplitude modulated time signal which is encoded by the lowering of the carrier frequency signal whereby the lowering has different impulse durations.
Conventional time signal receivers as for example described in the German Patent Publication DE 35 16 810 C2, receive the time signal as transmitted by the time signal transmitter in amplitude modulated form. These conventional receivers provide at their output a demodulated signal in the form of impulses having different durations. This demodulation takes place in real time, more specifically an impulse of a different duration appears per second at the output of the receiver corresponding to an idealized time signal as shown in FIG. 2. The time information is thus available in coded form based on the impulses of different time durations. The time signal receiver supplies these impulses of different time durations to a microcontroller connected to the output of the time signal receiver. The microcontroller evaluates the impulses and decides, based on the pulse duration, whether a logic bit value of “1” or “0” is to be allocated to the respective pulse or impulse. To perform this bit value allocation, first the beginning of a second of a respective time frame of the time signal is determined. Once the beginning of a second is known, it is possible to determine based on the pulse duration whether the bit value “1” or “0” applies. The microcontroller accepts in the following all 59 bits of a minute and determines on the basis of the bit encodings of a respective second impulse which precise time and date are present. This evaluation of the precise time and date is however only possible if the 59 second bits of a minute have been recognized without any ambiguity so that a “0” or a “1” can be allocated unambiguously to each of these second bits. Such a method for ascertaining the beginning of a second in the signal of a time signal transmitter is for example described in the German Patent Publication DE 195 14 036 C2.
In connection with the above mentioned German Patent Publication DE 195 14 036 C2 there exists a problem in that interfering signals may be superimposed on the time signals. Such interfering signals occur due to interfering fields generated by electrical or electronic equipment. FIG. 3(a) of said German Patent Publication shows a measurement of a time signal on which an interference signal is superimposed. Such a signal becomes available at the output of the receiver. FIG. 3b shows, compared to FIG. 3a, the respective time signal as it is transmitted by the time signal transmitter without any superimposed interfering signals. Depending on the type and size or volume of the interfering signals it may happen that the reception of the time signals is impaired. In such a case it is, however, necessary that the time signal is transmitted and received for a full minute until a correct reception of the time signal is possible so that the 59 second bits for determining the correct time and the correct date are available. Following such a situation, the previous result is compared with a second reception free of disturbances for one minute based on a complete time signal telegram to perform a plausibility test. When it is ascertained that coincidence or rather conformity exists between the two received time signal telegrams the information can be decoded and transformed into the time information.
Disturbances are particularly substantial in environments which are disturbance prone, for example in large cities, in and around large industrial plants, and in offices where there is a large number of data monitors and computer devices. In these facilities the disturbances are especially large and are called “disturbance fog”. Due to this disturbance fog it is frequently possible only after a very long time to have a correct reception of the time signal telegram. As a result, the time signal receivers must remain activated for a respectively long time. In connection with time signal receivers that are operated for example by a battery or an accumulator, this situation leads to a rapid consumption of the available energy supply.
Frequently a reception free of disturbance is available only during night hours. This means that a new start of the time signal receiver is possible only during that night time or only the following day when an exact time signal is available. This may happen for example when batteries need to be exchanged.
Further problems have been found to exist as follows. A microcontroller or processor is used in the radio controlled clock for decoding the time signal. The microcontroller which receives the output signal from the time signal receiver is typically constructed as a four-bit microcontroller to keep costs down. Such a microprocessor comprises a rather small memory of about two kBytes. This memory is used primarily for the program of the microcontroller. The program in turn serves primarily for the treatment of the disturbances and of different second impulses. The microcontroller in conventional clocks is substantially completely occupied with evaluating the received, disturbed second impulses. During this time the microcontroller is not available for solving other problems.
A very substantial knowledge regarding the occurrence of these disturbances during the reception of the time signal is necessary especially for a time signal on which disturbances are superimposed, as shown particularly in FIG. 3a of German Patent Publication DE 195 14 036 C2. Based on the special knowledge, the respective programs of the microcontrollers are so developed that a sufficiently certain evaluation of the time signal is possible even in an environment where relatively large disturbances exist. These programs are currently optimized with regard to the treatment of disturbances and thus determine substantially the quality of conventional radio controlled clocks.
Moreover, due to limited memory space it is possible to use only very compact programs. These programs are typically developed in an assembler programming language in order to achieve a high quality and certainty in the evaluation of the second impulses and in order to realize a very intelligent recognition of the disturbing signals. For these reasons, an exceptionally high or large know-how is necessary for the programming in order to solve the problems that must be met by the receiver technology and the limiting conditions outlined above. These problems particularly involve the influence of disturbances on the precision of the time signals and further problems mainly derived from the long wave technology and analog technology. Additionally, these programs are supposed to be programmed to be very compact so that the memory space requirement becomes as small as possible. However, at this time there are hardly any programmers capable of developing such respective programs while maintaining or satisfying the above mentioned limiting conditions. The quality of such programs, however, determines substantially the quality of a radio controlled clock.
The following publications provide further background information regarding radio controlled clocks and receiver circuits for receiving time signals. Reference is made in this context to German Patent Publications: DE 198 08 431 A1; DE 43 19 946 A1; DE 43 04 321 C2; DE 42 37 112 A1 and DE 42 33 126 A1. With regard to the synchronization of a time signal, reference is made to DE 298 13 498 U1 and DE 44 03 124 C2. Regarding the information retrieval and processing of time information from time signals, reference is made to DE 195 14 031 C2; DE 37 33 965 C2 and EP 042 913 B1.